Building corners: How to connect 2 walls with a straight corner
3 methods (or surely more):

1/Bridge edge loops (1):


Select the corresponding vertices
CtrlE then bridge edge loops
Select edges from bottom to top CtrlAltRMB
Scale 0 using individual origins as pivot
Remove doubles
Select the corner
Scale by 2 for X and Y from the cursor placed at the inner corner




2/Bridge edge loops (2):


Select the corresponding vertices
CtrlE then bridge edge loops
Set number of cuts to 1
Set interpolation to blend surface
Set smoothness to sqrt(2)




3/Rotation


Select the wanted vertices on one side
Place the cursor at the inner corner
Set the pivot to 3D cursor
Rotate 45 degrees
Scale sqrt(2) for X and Y
Do the same for the other side